Electronic – LM317 running digital pulses. Results are Noisy

linearlm317voltage-regulator

The set up of my experiment so far:

A. I use a simple LM317 linear voltage regulator to supply constant voltage of +5V to Vdd of my Laser driver circuit.

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B. Also I use another LM317 linear voltage regulator to supply voltage Vlda ( Up-to 12 volts ). Just the same way I shown in the previous figure , the source is from a 12V lead acid battery with 7.2 AmpHour rating.

C: The circuit of the Laser Driver:

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** Questions:**

A : I don't get a clear pulse waveform and it seems very noisy. This is not the case if I directly supply voltage from a battery without the LM317 in between.

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B : What is the difference between AGND ( analog ground ) and just Ground ( please see the highlighted areas in Laser Driver circuit ). I have shorted them as of now.

Best Answer

ICs that differentiate analog and digital ground generally do to to separate 'noisy' power circuitry from 'quiet' analog or digital circuitry. This allows you to keep ground places separated in the layout and allows for a single-point connection between power and signal ground, which is optimal from a noise standpoint.

The LM317 is not a low-dropout type of linear regulator. The attached datasheet has a set of curves on page 7 called "dropout voltage" that illustrates this point:

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You're trying to control an output that's within 1 volt of input. The LM317 is not an appropriate choice for this function - it is not regulating anything, as earlier comments have stated.

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